1. Field of the Invention
The present invention relates to the field of marine seismic data acquisition methods. More particularly, the invention relates to methods for acquiring high quality, towed-array marine seismic surveys.
2. Discussion of Related Art
A marine seismic acquisition survey typically involves one or more vessels towing at least one seismic streamer through a body of water believed to overlie one or more hydrocarbon-bearing formations. As those in the art having the benefit of this disclosure will appreciate, a “marine” survey may be performed not only saltwater environments, but also in fresh and brackish waters. The term “marine” is not limited to saltwater as used in this context by those in the art.
WesternGeco L.L.C. currently conducts high-resolution Q-MARINE™ surveys, in some instances covering many square kilometers. A survey vessel known as a Q-TECHNOLOGY™ vessel may conduct seismic surveys towing multiple 1,000 m-10,000 m cables with a crossline separation of 25 m-50 m, using the WesternGeco proprietary calibrated Q-MARINE™ source. “Q” is the WesternGeco proprietary suite of advanced seismic technologies for enhanced reservoir location, description, and management. For additional information on Q-Marine™, a fully calibrated, point-receiver marine seismic acquisition and processing system, as well as Q-Land™ and Q-Seabed™, see http://www.westerngeco.com/q-technology.
In many areas of the world hydrocarbon reservoirs located in structurally complex areas may not be adequately illuminated even with advanced towed-array acquisition methods. For example, the shallow, structurally complex St. Joseph reservoir off Malaysia produces oil and gas in an area that poses many surveying and imaging challenges. Strong currents, numerous obstructions and infrastructure, combined with difficult near-surface conditions, may hinder conventional survey attempts to image faults, reservoir sands, salt domes, and other geologic features.
To achieve high density surveys in regions having a combination of imaging and logistical challenges, a high trace density and closely spaced streamers may be used. However, this presents the potential of entangling and damaging streamer cables and associated equipment, unless streamer steering devices are closely monitored and controlled. Wide-azimuth towed streamer survey data is typically acquired using multiple vessels, for example: one streamer vessel and two source vessels; two streamer vessels and two source vessels; or one streamer vessel and three source vessels. Many possible marine seismic spreads comprising streamers, streamer vessels, and source vessels may be envisioned for obtaining wide- or rich-azimuth survey data.
Several wide- or rich-azimuth techniques are known to the art. Cole, R. A. et al., “A circular seismic acquisition technique for marine three dimensional surveys”, Offshore Technology Conference, OTC 4864, May 6-9, 1985, Houston, Tex., described a concentric circle shooting scheme for obtaining three dimensional marine survey data around a sub-sea salt dome. Another technique was described by Cole, R. A et al., in the paper presented at the SEG conference in 1988 “Three dimensional marine data acquisition using controlled streamer feathering”, where overlapping circles acquisition is proposed. The concentric circle technique could be useful perhaps when the location of the subsurface feature, like salt dome, is known. The overlapping circles technique could offer an increase in acquisition efficiency versus conventional acquisition.
While the Q suite of advanced technologies for marine seismic data acquisition and processing may provide detailed images desired for many reservoir management decisions, including the ability to acquire wide- and/or rich azimuth data, the ability to acquire higher quality marine seismic data with less cost, or to increase the fold while also increasing the diversity of azimuth and offset, are constant goals of the marine seismic industry and would be viewed as advances in the art.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.